1. Field of the Invention
The present invention relates to a method of coating metal strip with phosphate film, in particular steel strip for deep drawing, during a thermal treatment. The range of applications of the process which consists in coating a metal sheet, in particular a steel sheet, with a protective film, such as a phosphate film, is very wide; for example this film may be used as:
a temporary protection of the surface between manufacture and use: PA0 a base layer which is advantageous for subsequent adhesion of a final protective layer: PA0 a preparatory layer before painting: PA0 a layer which is advantageous for adhesion of the lubricant before forming.
2. Description of the Prior Art
These multiple applications only differ from one another by the thickness and the type of the protective film to be deposited, in particular the phosphate film.
We have already advocated a method for continuous thermal treatment of steel strip elsewhere. The method consists in heating the sheet to a temperature higher than its recrystallization temperature and then cooling it rapidly by immersing it in an aqueous bath held at a temperature higher than 75.degree. C. and preferably substantially at its boiling point.
We have also disclosed the possibility of combining this thermal treatment with a subsequent surface treatment designed to coat the strip with a metallic or phosphate film.